Hemipelagic and Pelagic Basin Plains - Trace Fossils as Indicators of Sedimentary Environments - page 671

sediments, the production of trace fossils ceases at lower C org values than in
oxygenated environments; mainly biodeformational structures are produced
instead. Off NW Africa, for instance, under fully oxygenated conditions, bio-
deformational structures dominate the ichnofabric above

2% C org ( Wetzel,
1981, 1991 ). In contrast, off central Vietnam, this threshold value is in the range
of 0.5-0.7% C org during periods of lowered oxygenation ( Wetzel et al., 2011 ).
In addition to the benthic-food content and oxygenation, other aspects need to
be taken into account when investigating ichnofabrics; especially for mud rocks,
the original sediment consistency, the role of biodeformational structures, and
the degree of compaction need to be considered. Themore enriched amuddy sed-
iment is in organicmatter, themore water-rich it tends to bewhen deposited (e.g.,
Keller, 1982 ). A rather soft consistency, however, favors the production of bio-
deformational structures (e.g., Lobza and Schieber, 1999; Wetzel et al., 2011 ).
The higher the porosity of a mud, the higher is the chance that clay minerals
become reoriented perpendicular to the pressure during compaction (e.g.,
Mitchell, 1993 ). In this way, the originally formed biodeformational structures
may become macroscopically pseudolaminated when compacted. Another
aspect is that the biodeformational structures are rarely mentioned in ichnofabric
analysis (cf. Bromley and Ekdale, 1984; Savrda and Bottjer, 1986 ).

3.4 Settings Affected by Bottom Currents
The effects of deep-sea bottom currents on the habitat have been studied mainly
in modern settings that are influenced by contour currents (for summary, see
Fig. 11 ). Such areas have been investigated in great detail (e.g., Nowell and
Hollister, 1985; Stow et al., 2002 , and references therein). The mean flow veloc-
ity of contour currents is in the range of 10-30 cm/s ( McCave et al., 1980 ). In
fact, the current speed can fluctuate within a few days to several weeks from
quiescence to benthic storms ( 70 cm/s; Richardson et al., 1981 ). Contour cur-

rents may therefore rework the sea floor and take-up particles that are settling
down ( Fig. 8 ). Thus, such currents can carry a considerable amount of fine
grains including particulate organic matter (e.g., McCave, 1985; Thomsen
et al., 2002) and supply food to deep-marine biota (e.g., Thistle et al., 1985 ).
Therefore, the local benthos exhibits a higher population density and biomass
than in adjacent, more tranquil settings (e.g., Thistle et al., 1985 : 103).
At the contour-current affected HEBBLE (high energy benthic boundary
layer experiment) site off Nova Scotia, the macro- and meiofauna are on average
extremely abundant for that depth and comparable to sites 2,000 m shallower
( Thistle et al., 1985; Thomson et al., 2000 ). Because of the repeated physical
disturbance, the fauna resembles that found in recolonization experiments
( Thistle et al., 1991 ). With respect to behavioral groups, ordinary suspension
feeders are not abundant, probably as their filter apparatus can easily be plugged
when the suspension concentration is very high for some time ( Thistle et al.,
1991 ). Taxa, however, that passively extract drifting particles by maintaining

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